While Salesforce is not a pure model of a paid API, they're not exactly unpopular.
When original Google Maps API came out, developers literally were begging Google to introduce paid APIs for some of the stuff they were writing. I went to a maps meetup in Sunnyvale, and that quickly became the subject of the night.
Developers understand that paid APIs
(a) offer SLAs
(b) usually are not changed drastically, which for maps scenario allows embedding them into hardware
(c) come with a phone number for emergency support
(d) hint at what developer's pricing schema should be (e.g. if API charges a monthly subscription fee, makes sense to build the product around that, if it's number of queries to the API, makes sense to incorporate the final product into a few options for light, medium and heavy users).
When original Google Maps API came out, developers literally were begging Google to introduce paid APIs for some of the stuff they were writing. I went to a maps meetup in Sunnyvale, and that quickly became the subject of the night.
Developers understand that paid APIs
(a) offer SLAs
(b) usually are not changed drastically, which for maps scenario allows embedding them into hardware
(c) come with a phone number for emergency support
(d) hint at what developer's pricing schema should be (e.g. if API charges a monthly subscription fee, makes sense to build the product around that, if it's number of queries to the API, makes sense to incorporate the final product into a few options for light, medium and heavy users).